Tag: interferometer

To be part-wave and part-particle at the same time

September 3rd, 2021

Experimental physics is littered with freaky effects, often the product of obscure forces moving and changing objects in ways we don’t expect, but almost always leading to perfectly understandable conclusions. One notable exception – arguably the most notable, in fact – is the double-slit experiment.

Cut two narrow, parallel slits on an opaque sheet and shine light on them. If the conditions are right, you’ll see an interference pattern on the wall behind the sheet – the result, and proof, of the photons’ wavelike behaviour. But if you stick a small detector on each of those slits to track the movement of waves through each one, the interference pattern will be replaced by one or two small pricks of light on the wall – the result, and proof, of the photons behaving like particles and moving only through one slit or the other. Taken together, the experiment demonstrates the wave-particle duality of quantum objects (objects whose behaviour is dictated by quantum forces, as opposed to macroscopic objects that are dominated by classical forces).

In the more than two centuries since the first double-slit experiment, in 1801, many groups of scientists have modified it in different ways to elicit different aspects of the duality, and their implications for the study of the nature of reality. Anil Ananthaswamy’s 2018 book Through Two Doors At Once wends its way through this history, at each step stopping to identify more and more strange discoveries that have only complicated, instead of simplified, the behaviour of particles like photons. One especially weird possibility is contained in a thought-experiment called the Elitzur-Vaidman bomb tester.

Another is contained in a series of famous thought-experiments that American physicist John Wheeler proposed from the late 1970s. Essentially, he asked if each photon could make a ‘decision’ about whether it would travel as a particle or as a wave based on the experimental setup in front of it, if this decision happened in a certain time frame, and if an observer could anticipate this decision-making moment and interfere with it. As bizarre as this sounds, physicists have been able to set up experiments whose results have been consistent with some of Wheeler’s hypotheses.

For example, say you shine a laser at a beam-splitter.

An aluminum coated beam-splitter, designed to be 80% reflective. Caption and credit: Zaereth, public domain

The beam is split in two perpendicular directions; let’s call them A and B. A is made to bounce off a mirror by 90º and moves to a point, which we’ll call P. B is also turned 90º by a mirror in its path and directed to P. If there is a detector at P, physicists have observed a prick of light – indicating both A and B beams were composed of particles. But if there is another beam-splitter at P, then the combined A and B beams are split once again into two beams – and one of them has shown an interference pattern. If A and B were composed of particles until they struck the detector or splitter at P, where did the waves come from? Or, according to Wheeler’s hypothesis, did the photons travelling as part of A and B anticipate that there would be a splitter instead of a detector at P, and decided to become waves? We don’t know. Specifically, there are different interpretations of the experiment’s outcomes that try to make sense of what happened, but we don’t have objective data that supports one exact possibility, in a classical sense.

Wheeler himself concluded that there are no phenomena in the natural universe that are independent of their observations. That is, until you observe something (quantummy) happening, Wheeler figured it wouldn’t have happened (at least not the way you think it did). But more importantly (for this post), both Wheeler’s ideas and the experiments that physicists used to elucidate wave-particle duality kept the focus on the particle, the observer and the test setup. A new study by scientists in the US may complicate this picture even more: they’ve reported evidence that the source of the particles could also influence how the particles behave in an experiment.

Theoretical physicists have anticipated such a finding. For example, one paper published in February 2020 said that when its authors set out to quantify the extent to which a setup would produce an interference pattern or pinpricks of light, they found a simple mathematical relationship between this measure and the purity of the photon source. In the new study, simply put, physicists flashed a specifically tuned laser onto two crystals, of lithium niobate. The crystals then emitted two photons each, which the physicists called the ‘signal’ and the ‘idler’. They directd the signal photons from both crystals to an interferometer – a device that splits a beam of light into two and recombines them to produce an interference pattern – to observe the characteristic proof of wave-like behaviour; they also directed the two idler photons to two detectors, to confirm their particle-like behaviour.

Each pair of signal and idler photons produced by each crystal would be entangled. Wikipedia: “Quantum entanglement is a physical phenomenon that occurs when a group of particles are generated, interact or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance.” One implication of this relationship is that if we discover, or observe, one of two entangled particles in a certain quantum state, we can determine the state of the other particle without observing it.

In their experiment, the physicists effectively mapped source purity with “the likelihood that a particular crystal source will be the one that emits light” (source). That is, by increasing or decreasing the chances of one of the two crystals emitting photons – by adjusting the strength of the incident laser – the physicists could control the value of the source purity they needed to plug into the equation. They found that when one of the crystals became very likely to emit paired photons, the interference pattern became very feeble – i.e. the photons at the interferometer were behaving like particles. The interference pattern was sharpest when both crystals were equally likely to emit paired photons. These results confirmed the (theoretical) findings of the February 2020 paper, but the physicists were able to do one better.

The experimental setup. BS: beam-splitter; DM: dichroic mirror; PPLN: a lithium niobate crystal; i1, i2: idler photons; s1, s2: signal photons. Source: DOI: 10.1126/sciadv.abi9268

The February 2020 paper posited that source purity (µ), interference visibility (V) and ‘particle location distinguishability’ (D) were related thus: V² + D² = µ². The new paper also found that µ² = 1 – E², where E is a measure of the extent of entanglement between an idler photon and the detector detecting it. This is new, and we don’t yet know how other physicists will exploit it to delve even more into the seemingly bottomless pit that is wave-particle duality. Equally, the experiment also demonstrates, according to Xiaofeng Qian, one of the authors of the February 2020 paper, that a “quantum particle can behave simultaneously, but partially, as both” wave and light.
The molecule that was also a wave

December 12th, 2019

According to the principles of quantum mechanics, you’re a wave – just like light is both a particle and a wave. It’s just that your wavelength is so small that your wave nature doesn’t matter, and you’re treated like a particle. The larger an object is, the smaller its wavelength, and vice versa. We’re confused about whether light is a particle or a wave because photons, the particles of light, are so small and have a measurable wavelength as a result. Scientists know that electrons, protons, neutrons, even neutrinos have the properties of a wave.

But while the math of quantum mechanics says you’re a wave, how can we know for sure if we can’t measure it? There are two ways. One, we don’t have any evidence to the contrary. Two, scientists have been checking if larger and larger particles, as far as they can go, exhibit the properties of a wave – and at every step of the way, they’ve come up with positive results. Both together, we have no reason to believe that we’re not also waves.

Such tests reaffirm the need for quantum mechanics to understand the nature of reality because the rules of classical mechanics alone don’t explain wave-particle duality.

On September 23, scientists from Austria, China, Germany and Switzerland reported that they had measured the wavelength of a group of molecules called oligoporphyrins. Specifically, they used “oligo-tetraphenylporphyrins enriched by a library of up to 60 fluoroalkylsulphanyl chains”. Altogether, they consisted “of up to 2,000 atoms”, becoming the heaviest object directly known to exhibit wave-like properties.

The molecule in question. DOI: 10.1038/s41567-019-0663-9

According to the scientists’ peer-reviewed paper, the molecules had a wavelength of around 53 femtometers, about 100,000-times smaller than the molecules themselves.

* * *

We have known since at least the 11th century, through the work of the Arab scholar Ibn al-Haytham, that light is a wave. In 1670, Isaac Newton propounded that light is made up of small particles, and spent three decades supplying evidence for his argument. His push birthed a conflict: was light wave-like or made up of particles?

The British polymath Thomas Young built on the 17th century Dutch physicist Christiaan Huygens to devise an experiment in 1801 that definitively proved light was a wave. It is known widely today as the Young’s double-slit experiment. It is so simple even as its outcomes are so immutable that it has become a mainstay of modern tests of quantum mechanics. Physicists use upgraded versions of the experiment to this day to study the nature and properties matter-waves.

(If you would like to know more, I highly recommend Anil Ananthaswamy’s biography of this experiment, Through Two Doors At Once; here’s an excerpt.)

In the experiment, light from a common source – such as a candle – is allowed to pass through two fine slits separated by a short distance. A sheet of paper sufficiently behind the slits then shows a strange pattern of alternating light and dark bands instead of just two patches of light. This is because light waves passing through the two slits interfere with each other, producing the famous interference pattern. Since only waves can interfere, the experiment shows that light has to be a wave.

An illustration of the double-slit experiment from ‘Though Two Doors At Once’ (2019).

The particulate nature of light would get its proper due only in 1900, when Max Planck stumbled upon a mathematical inconsistency that forced him to conclude light had to be made up of smaller packets of energy. It was the birth of quantum mechanics.

* * *

The international group’s test went roughly as follows: the scientists pulsed a laser onto a glass plate coated with the oligoporphyrins to release a stream of the molecules; collected them into a beam using collimators; randomly chopped the beam into smaller bits; passed each bit through diffraction gratings to split it up; then had the two little beams interfere with each other. Finally, they counted the number of molecules striking the detector while the detector registered the interference pattern.

They had insulated the whole device, about 2m long, from extremely small disturbances, like vibrations, to prevent the results from being corrupted. In their paper, the scientists even write that the final interference pattern was blurred thanks to Earth’s rotation, and which they were able to “compensate for” using effects due to Earth’s gravity.

A schematic diagram of the experimental setup. The oligoporphyrins move from left to right as the experiment progresses. The results of the counter are visible in a diagram above the right-most component. DOI: 10.1038/s41567-019-0663-9

To ascertain that the pattern they were seeing on the detector was in fact due to interference, the scientists performed a variety of checks each of which established a relationship between the shapes on the detector with the properties of the components of the interferometer according to the rules of quantum mechanics. They were also able to rule out alternative, i.e. classical, explanations this way.

For example, the scientists fired a laser through the cloud of molecules post-interference. Each molecule split the laser light into two separate beams, which recombined to produce an interference pattern of their own. This way, scientists could elicit the molecules’ interference pattern by studying the laser’s interference pattern. As they varied the laser power, they found that the visibility distribution of the molecules more closely matched with quantum mechanical models than with classical models, confirming interference.

The solid blue line indicates the quantum mechanical model and the dashed red line is a classical model, both scaled vertically by a factor of 0.93. The shaded areas on the curves represent uncertainty in the model parameters, and the dotted lines indicate unscaled theory curves. DOI: 10.1038/s41567-019-0663-9

What these scientists have achieved isn’t only a feat of measurement. Their findings also help refine the border between the classical and the quantum. The force of gravity governs the laws of classical mechanics, which deals with macroscopic objects, while the electromagnetic, strong nuclear and weak nuclear forces rule the microscopic world. Although macroscopic and microscopic objects occupy the same universe, physicists haven’t yet understood how classical and quantum mechanics can be combined into a single theory.

One of the problems standing in the way of this union is knowing where – and how – the macroscopic world ends and the microscopic world begins. So by observing quantum mechanical effects at the scale of thousands of atoms, scientists have quite literally pushed the boundaries of what we know about how the universe works.

Tag: interferometer

To be part-wave and part-particle at the same time

The molecule that was also a wave